linux-kernel - Re: [PATCH 8/9] psi: pressure stall information for CPU, memory, and IO

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Date:   Tue, 28 Aug 2018 14:30:17 -0700
From:   Randy Dunlap <rdunlap@...radead.org>
To:     Johannes Weiner <hannes@...xchg.org>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Tejun Heo <tj@...nel.org>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Daniel Drake <drake@...lessm.com>,
        Vinayak Menon <vinmenon@...eaurora.org>,
        Christopher Lameter <cl@...ux.com>,
        Peter Enderborg <peter.enderborg@...y.com>,
        Shakeel Butt <shakeelb@...gle.com>,
        Mike Galbraith <efault@....de>, linux-mm@...ck.org,
        cgroups@...r.kernel.org, linux-kernel@...r.kernel.org,
        kernel-team@...com
Subject: Re: [PATCH 8/9] psi: pressure stall information for CPU, memory, and
 IO

On 08/28/2018 01:56 PM, Johannes Weiner wrote:
> On Tue, Aug 28, 2018 at 01:11:11PM -0700, Randy Dunlap wrote:
>> On 08/28/2018 10:22 AM, Johannes Weiner wrote:
>>> diff --git a/Documentation/accounting/psi.txt b/Documentation/accounting/psi.txt
>>> new file mode 100644
>>> index 000000000000..51e7ef14142e
>>> --- /dev/null
>>> +++ b/Documentation/accounting/psi.txt
>>> @@ -0,0 +1,64 @@
>>> +================================
>>> +PSI - Pressure Stall Information
>>> +================================
>>> +
>>> +:Date: April, 2018
>>> +:Author: Johannes Weiner <hannes@...xchg.org>
>>> +
>>> +When CPU, memory or IO devices are contended, workloads experience
>>> +latency spikes, throughput losses, and run the risk of OOM kills.
>>> +
>>> +Without an accurate measure of such contention, users are forced to
>>> +either play it safe and under-utilize their hardware resources, or
>>> +roll the dice and frequently suffer the disruptions resulting from
>>> +excessive overcommit.
>>> +
>>> +The psi feature identifies and quantifies the disruptions caused by
>>> +such resource crunches and the time impact it has on complex workloads
>>> +or even entire systems.
>>> +
>>> +Having an accurate measure of productivity losses caused by resource
>>> +scarcity aids users in sizing workloads to hardware--or provisioning
>>> +hardware according to workload demand.
>>> +
>>> +As psi aggregates this information in realtime, systems can be managed
>>> +dynamically using techniques such as load shedding, migrating jobs to
>>> +other systems or data centers, or strategically pausing or killing low
>>> +priority or restartable batch jobs.
>>> +
>>> +This allows maximizing hardware utilization without sacrificing
>>> +workload health or risking major disruptions such as OOM kills.
>>> +
>>> +Pressure interface
>>> +==================
>>> +
>>> +Pressure information for each resource is exported through the
>>> +respective file in /proc/pressure/ -- cpu, memory, and io.
>>> +
>>
>> Hi,
>>
>>> +In both cases, the format for CPU is as such:
>>
>> I don't see what "In both cases" refers to here.  It seems that you could
>> just remove it.
> 
> You're right, that must be a left-over from when I described CPU
> separately; "both cases" referred to memory and IO which have
> identical formats. It needs to be removed:
> 
> diff --git a/Documentation/accounting/psi.txt b/Documentation/accounting/psi.txt
> index e051810d5127..b8ca28b60215 100644
> --- a/Documentation/accounting/psi.txt
> +++ b/Documentation/accounting/psi.txt
> @@ -35,7 +35,7 @@ Pressure interface
>  Pressure information for each resource is exported through the
>  respective file in /proc/pressure/ -- cpu, memory, and io.
>  
> -In both cases, the format for CPU is as such:
> +The format for CPU is as such:
>  
>  some avg10=0.00 avg60=0.00 avg300=0.00 total=0


OK.  However, after reading patch 9/9, I thought that the "both cases"
could possibly mean the files in /proc/pressure/ and the files in
cgroup ({cpu,io,memory}.pressure).

-- 
~Randy